Apparently, if you go to Castle Volkihar before going to the Dawnguard the questline becomes unplayable. My friend encountered this problem and now his character that was going to be on the vampire side is broken. Does anyone know a fix for this and if not at the very least I hope Bethesda takes notice of it.

1. Reload a previous Save.
2. You must first go to Fort Dawnguard. This does not mean you've joined the faction.
3. The choice of siding with Dawnguard or Volkihar is 3 quests into the dlc.

Hope that helps